1. Field
The present invention relates to an optical transmission/reception system, an optical transmitter, an optical receiver, and an optical transmission/reception method. The present invention is applied to, e.g., a polarization multiplexing optical communication system which performs modulation with an independent signal sequence for each polarization component.
2. Description of the Related Art
In recent years, attention has been focused on polarization multiplexing communication capable of multiplying bit rate per baud in an optical communication system. Utilization of adaptive polarization separation signal processing is favorable for the separation of a polarization multiplexed signal. In adaptive polarization separation signal processing, digital signal processing is applied to an analog/digital conversion technology and a digital coherent technology. The adaptive polarization separation signal processing adaptively estimates polarization properties of a transmission path and demodulates the polarization multiplexed signal.
It is to be noted that the polarization properties on the transmission path in the optical communication system fluctuate with time and cause degradation in communication quality. Examples of the properties which fluctuate with time include a status of polarization (SOP), polarization-mode-dispersion (PMD), a polarization-dependent-loss (PDL), and a polarization-dependent-gain (PDG). These polarization-property fluctuations lower performance of the polarization multiplexing communication system.
It is known that the degradation caused by polarization dependence such as the PMD, the PDL, and the PDG may be reduced by scrambling the polarization. In a DWDM (Dense Wavelength Division Multiplexing) system, it is possible to mount a module which collectively performs optical polarization scrambling with respect to DWDM signal light in a relay node after the DWDM. As a result, there are cases where this arrangement achieves advantages in terms of reductions in cost, device size, and power consumption, etc. when compared with the case in which an optical polarization scrambler is individually mounted in each of optical transmitters.
In addition, in recent years, in order to cope with waveform distortion on a transmission path, it is proposed to reduce the signal degradation caused by a linear effect, a non-linear effect, and a polarization-dependent effect on the transmission path by performing signal processing on the transmission side of the optical communication system to preliminarily convert (pre-equalize) a transmission signal.
[Non-Patent Document 1] T. Pfau et al., “PDL-Tolerant Real-time Polarization-Multiplexed QPSK Transmission with Digital Coherent Polarization Diversity Receiver”, LEOS Summer Topical Meetings, 2007 Digest of the IEEE, 2007
[Non-Patent Document 2] T. Pfau, et al., “Ultra-Fast Adaptive Digital Polarization Control in a Realtime Coherent Polarization-Multiplexed QPSK Receiver”, OTuM3 OFC 2008 (San Diego, 2008)
[Patent Document 1] US Patent Application publication Number 2005/0226633 Specification
[Patent Document 2] US Patent Application publication Number 2006/0127104 Specification
In the above-described technology which collectively performs the optical region polarization scrambling with respect to the DWDM signal in the relay node, since the polarization scrambling is collectively performed for all channels, there are cases where a channel in which a penalty is increased by the polarization scrambling is included depending on a modulation method. Consequently, it is preferable to be able to appropriately determine whether or not the polarization scrambling is to be performed in accordance with each of the channels. However, a device performing the polarization scrambling in the optical region is burdensome in terms of the device size and cost, and it is therefore difficult to introduce the device in each of the channels.
In the technology (pre-equalization technology) which preliminarily converts the waveform of a transmission signal on the transmission side in the optical communication system, it is not possible to suppress the influence exerted by the PMD, the PDL, and the PDG on the transmission path which constantly fluctuate. Consequently, it is desired to be able to suppress the influence on reception signal quality exerted by the constantly fluctuating transmission path properties and maintain excellent reception signal quality.
In the adaptive polarization separation signal processing to which the conventional digital signal processing is applied, it has been possible to respond to and adaptively compensate for slow polarization state fluctuations. However, it is not easy to respond to the polarization property having high fluctuation speed such as, e.g., the polarization property fluctuating at intervals of not more than 1 millisecond, using the adaptive polarization separation signal processing. This means that it becomes difficult to perform the adaptive polarization separation signal processing on the reception side when high-speed polarization scrambling is performed in order to suppress the influence by the polarization properties which fluctuate with time.